Elimination of the risk of particul

Elimination of the risk of particulate contamination in the SG channel heads and decreased radiation fields in the SGs, due to isolation of the SGs prior to forced oxidation.One disadvantage of this shutdown strategy is that the RCS fluid in the SG loops is not cleaned up during shutdown.There is potential for high activity water to be trapped in the SG piping when the loops are isolated or drained.However, activity levels in the SG loops may be expected to be low if the RCS volume is reduced prior to forced oxidation, given that the majority of the release occurs upon oxidation.The use of this shutdown strategy similarly leads to more residual deposits remaining on the core after shutdown and may be used in conjunction with ultrasonic fuel cleaning(UFC)or other crud removal techniques.In this section, the metrics of shutdown performance used for the purpose of this analysis are described.The metrics of shutdown performance are as follows5:For each metric, the methodology used to quantify shutdown performance(either directly or through indirect indicators of shutdown performance)is also described.Data for the analyses were drawn from the CMA database, when available, and provided directly by plant personnel.Basis for Data AnalysisDuring shutdown, the change in concentrations in the coolant can be simplified to the schematic shown in Figure 4-1.In this simplification, material is released into the coolant from the deposits on the fuel and removed(i.e., cleaned up)by the letdown(CVCS)purification system.The change in concentration in the coolant can be expressed mathematically by Equation[4-1].d(.MRCSCRCS)— m C — m C+Cletdown letdown,return letdown letdown releasewhere MRCS is the coolant mass being actively circulated and cleaned up, CRCS is the concentration in the coolant, mietdown is the letdown mass flow rate, Cietdown,return is the concentration in the letdown return, Cletdown is the concentration in the letdown intake, and Rrelease is the rate at which material is released into the coolant.6Available evidence indicates that there is little release during shutdown from the out-of-core surfaces.However, as these surfaces are the origin of corrosion products, there must be some dynamic interchange between the coolant and these surfaces.However, the models used in this analysis assume that this exchange is negligible during shutdown compared to the release from the fuel surfaces.RCS CoolantSimplified Schematic of Deposit Flow in the RCS Coolant during ShutdownIt is also assumed that the RCS is well mixed7 such that the concentration in the letdown flow is approximately equal to the concentration throughout the RCS.In order to justify the well mixed assumption, a comparison must be made between the time scales of nickel removal via letdown purification and RCS volume turnover.This can be accomplished by comparing the letdown flow rate to the RCS circulation flow rate, since both are processing the same volume of fluid.In the worst case scenario(high letdown purification flow rate and low RCS flow(e.g., operation of only RHR pumps)), the RCS flow rate is at least an order of magnitude greater than the letdown flow rate, which implies that the concentration at the reactor leg of the letdown inlet is approximately equal to the concentration at the reactor leg of the letdown outlet.Additionally, it is assumed that the letdown purification is perfect, such that the concentration in the letdown return is negligible8[9].Under these two assumptions, Equation[4-1]simplifies to Equation[42].An expression for the release rate of deposits from the fuel over a short time interval can be derived from Equation[4-2]by integrating over time.The release rate from time ti-1 to t can be approximated by the following equation:where Qletdown is the letdown volumetric flow rate, Qletdown,avg is the average of Qletdown at time ti and time ti-1, and VRCS is the RCS volume.The methodology for calculating shutdown metrics described in the following sections is based on the model of the RCS described by Equation[4-2].